Among the lessons driven home by Katrina[1] is that a coastal city below sea level is a city at risk. Levees can only do so much; the only guarantee of protection from the ocean is to be above it.

New Orleans was not this way originally. The first settlers would not have put themselves in a place where levees and pumps were required just to throw up a cabin; the land on which the city was built was originally above sea level. But the construction of the city and the prevention of the normal deposition of silt which built the land halted the process which kept it that way, and the pumping of groundwater plus the normal consolidation of silt left the city sinking. I've read that some parts of the city are 16 feet below sea level, and subsidence runs as fast as a half-inch per year.

It would be desirable to have a city that is once again above the water. To do this, the land beneath it would have to be built up by an average of about eight feet and then continuously over time. But how to do that?

Let it flood, let it flood, let it flood

One possible method is to let floods do it as they did before. One way engineers have handled flooding is to lift buildings out of harm's way and otherwise let nature take its course. In some areas, houses have been raised on pilings to allow floodwaters to pass harmlessly beneath; the new basement area is enclosed with blow-out panels which prevent harm from the force of flowing water.

Allowing annual floods to pass over and through an "uplifted" New Orleans would be an inconvenience, to be sure. While it might be feasible to commute in boats during the wet season, clearing mud from streets afterward would be a chore perhaps more onerous than snow removal and most typical landscaping would suffer. Further, adding up to sixteen feet of material to bring the lowest parts up to sea level would take a very long time and require several intermediate reconstructions of roadways. Provision of utilities to buildings with "wet feet" would also be a challenge, and annual or more frequent interruptions in service would seriously affect the habitability of even dry buildings in the mean time. It would be better to do the job more quickly than that, or at least in just one step.

90% of everything is crap

Or so was the soil beneath habitations until fairly recently. In an ancient city, subsidence would not have been so much of a problem. Without garbage removal, much of the ephemeral goods which came to a city tended to remain there as refuse. It built up in layers, incorporating all sorts of bits of interest to archæologists, and elevated the entire city over time. As first floors became cellars, new buildings were constructed above the levels of the old and the entire city climbed with time. A sufficiently slovenly populace might keep pace with subsidence and maintain itself above sea level in a river delta even without the benefit of flood-borne silt.

We would never accept this today. On the other hand, it would not be necessary (or even desirable) to use garbage or compost. The river delivers huge quantities of silt each year to the area, which the Army Corps of Engineers spends an equally huge amount of effort to dredge out of shipping channels. Once dried somewhat, this silt forms excellent soil. Why not use it?

It appears likely that many of the lowest areas of the city are destroyed, perhaps beyond repair. Absent billions for higher levees they will be at high enough risk to perhaps be uninsurable, and will take tens of billions to reconstruct in any event. It makes little sense to spend so much money just to put so much property and so many people back in harm's way; the rebuilt areas should be engineered to remain safe for the useful lives of the structures. This requires an immediate lift and the capability for on-going elevation. So here is my proposal for a reconstructed New Orleans:

Buildings too damaged to be worth saving would be razed. Buildings to be saved would be jacked off their foundations, new foundation piers installed beneath, and elevated above what is to be the new ground level; in the lowest areas this would be at least 16 feet and could be much more than that. The attachments between the piers and the beams would not be permanent; they would be detachable at any time so that shims could be added to elevate the building further. At the new ground level and perhaps at intervals above, tie beams would be added to keep the piers in alignment and allow for the attachment of cross-bracing; this would keep the piers from shifting under horizontal loads such as hurricane winds.

The process would go something like this:

(images unavailable due to Blogger bug, will try to add later)

Regions due to be razed in their entirety could be filled to a considerable height above sea level and then allowed to consolidate before beginning new construction; this would give better support to new road surfaces, driveways and sidewalks. Roadways might be engineered like boardwalks, made of floating segments loosely connected to each other and adjusted by additions of semi-liquid material beneath. Foundation piers should be extended to well-settled material to prevent the need for frequent levelling. As subsidence continued, periodic road work would be accompanied by less-drastic lifts of buildings on their piers, addition of spacers in the foundation posts, and filling of yards and other areas to maintain their proper elevation. As the city's soil sank, it would shim itself up to keep pace.

Would New Orleans be quite the same after this? Probably not, but even pre-Katrina it was not exactly the same as it was 50 years ago, or even 20. A shift from levee-building to whole-city elevation would be a radical change in process, but it would allow further change to proceed by increments rather than catastrophic jumps. It would allow for a great deal more stability and less worry. Isn't that what everyone wants?

Footnotes:

[1]: This lesson was only driven home this time. It was learned long ago, written up dozens of times, and published in everything from government disaster-preparedness studies to glossy popular magazines. Unfortunately, the people who had taken the money and assumed the responsibility to do something about it decided to procrastinate. (back)
¶ 9/03/2005 03:42:00 AM

Comments:

Of course, they could rebuild on higher ground.

I want to get my vote in early.

First New Orleans should not be rebuilt in toto. The historic and tourist areas should be leased out to Disney (or someone like them) and be turned into a theme park and tourist attraction.

The rebuilt city could include housing for the people who work there and second homes and condos. It could even include housing for some eccentrics and color types. It might have a permanent population of 250,000. It would be built behind new reinforced levies with more safety features.

Actually, levees could have protected it. The current levees were specifically built to withstand category 3 hurricanes. It was clear that they would not handle cat 5, but they could have been built to do so. I have the impression it would have cost about $2B.

A $50M planning study of an upgrade was defunded roughly 2 years ago by the Bush admin, for Iraq war and other homeland security expenses.

Spit out the kool-aid Nick. A planning study is the first step in a 10 to 20 year process. Besides, levee reinforcement is just doubling down on an unstable situation. The River wants to jump its channel. The city is subsiding. At some point N.O. would have sunk to far and the River would jump.

Ouch. Robert, did that feel like a personal attack? Let's not go there.

The point here is that the levee break in this situation was expected, predicted, and preventable. Was it reasonable to take the gamble that a cat 4 hurricane wouldn't hit? I don't know. But I should think that part of the planning process for the future of NO would be a re-examination of the previous decision making process. Was it a rational decision, or was the flooding of NO just the result of wishful thinking?

Perhaps NO in it's present form is untenable in the long run. On the other hand, the Netherlands have been living below sea level for centuries, so that alone isn't enough. My understanding is that they have much more sophisticated defenses against the ocean than dirt levees. I'd like to see a really good analysis of the cost/benefit of different alternatives....

..and the point about the defunded planning study is that that would have been the venue to do just such a re-examination. Instead, apparently we were just coasting along and deciding, by default, to just hope nothing would happen.

Ignoring problems until they explode has been the standard response of the Bush administration to everything. They are more concerned with ways to convert the taxpayer's money into assets for their cronies.

This country angers slowly, but this sort of malfeasance goes beyond mere incompetence. It takes a special kind of malevolence to remove money from measures intended to prevent predictable problems, and I am already calling it treason. I would not be surprised if this administration and many of its cronies wind up being charged, convicted and put before firing squads before the 2012 elections.

The people who just hoped that nothing would happen were the mayor and the governor who did not have a manditory evacuation plan in place despite the fact that they knew that 100,000 people would not be able to comply with an evacuation order due to age, poverty, illness etc.

Reinforcing levies would only compound the long term problem and postpone the inevitable.

Levees are a stopgap, which is why I'm suggesting that the city either be raised well above sea level or abandoned.

I had been under the impression that the area affected was a few square miles, but Wikipedia says that the land area is 180 square miles. (One wonders if this is just the city or if suburbs are included.) Filling this area to a height greater than a worst-case storm surge would take a lot of material; if the area which is currently 16 feet below sea level is filled to 22 feet above (the level of a worst-case storm surge), it would require 24320 acre-feet of material per square mile, or 39 million cubic yards per square mile. If this area was filled with silt at a rate of 20 cubic yards per minute (easily done with a system of buckets on a cable-car system) it would fill area at a rate of about 1/4 square mile per year.

If the city sinks at 1/2 inch per year, adding fill to keep it at the same level would require 43000 cubic yards per square mile per year, or 7.74 million cubic yards/year for all 180 square miles. This is an on-going pace of only about 15 cubic yards a minute.

Would the entire area need to be brought up to sea level? I'd think it would be advantageous to build up the coastal strip adjacent to the levees even higher than sea level - structures could be incorporated that would reinforce the sea wall, (ie elevated coastal highway), then the areas further inland wouldn't require as much fill. As for the amount of fill required...aren't they having a difficult time deciding where to put all the waste from "Katrina"? There are a lot of places with a lot of garbage that would rather not have a landfill in or near their community, even though there are environmentally safe methods of constructing landfills & making use of the methane gas, etc. What ever happened to the barges filled with garbage that once floated around out in the ocean? They should pull into the Port of New Orleans! How about an elevated coastal golf course...(http://www.apwa.net/Publications/Reporter/ReporterOnline/index.asp?DISPLAY=ISSUE&ISSUE_DATE=092004&ARTICLE_NUMBER=883)